[145]二叉树的后序遍历

给定一个二叉树，返回它的 后序 遍历。
示例:

输入: [1,null,2,3]  
   1
    \
     2
    /
   3 
输出: [3,2,1]

进阶: 递归算法很简单，你可以通过迭代算法完成吗？

/**
 * Definition for a binary tree node.
 * struct TreeNode {
 *     int val;
 *     TreeNode *left;
 *     TreeNode *right;
 *     TreeNode() : val(0), left(nullptr), right(nullptr) {}
 *     TreeNode(int x) : val(x), left(nullptr), right(nullptr) {}
 *     TreeNode(int x, TreeNode *left, TreeNode *right) : val(x), left(left), right(right) {}
 * };
 */
class Solution {
public:
    vector<int> postorderTraversal(TreeNode* root) {
                vector<int> res;
        if(root == nullptr){
            return res;
        }

        vector<int> left = postorderTraversal(root->left);
        vector<int> right = postorderTraversal(root->right);
        res.insert(res.end(), left.begin(), left.end());
        res.insert(res.end(),right.begin(), right.end());
        res.push_back(root->val);
        return res;
    }
};